There has been a growing interest in the manufacture and use of microfluidic systems for the acquisition of chemical and biochemical information. Techniques commonly associated with the semiconductor electronics industry, such as photolithography, wet chemical etching, etc., are being used in the fabrication of these microfluidic systems. The term, xe2x80x9cmicrofluidicxe2x80x9d, refers to a system or device having channels and chambers which are generally fabricated at the micron or submicron scale, e.g., having at least one cross-sectional dimension in the range of from about 0.1 xcexcm to about 500 xcexcm. Early discussions of the use of planar chip technology for the fabrication of microfluidic systems are provided in Manz et al., Trends in Anal. Chem. (1990) 10(5):144-149 and Manz et al., Avd. in Chromatog. (1993) 33:1-66, which describe the fabrication of such fluidic devices and particularly microcapillary devices, in silicon and glass substrates.
Applications of microfluidic systems are myriad. For example, International Patent Appln. WO 96/04547, published Feb. 15, 1996, describes the use of microfluidic systems for capillary electrophoresis, liquid chromotography, flow injection analysis, and chemical reaction and synthesis. A related patent, U.S. Pat. No. 5,942,443 assigned to the present assignee, discloses wide ranging applications of microfluidic systems in rapidly assaying compounds for their effects on various chemical, and preferably, biochemical systems. The phrase, xe2x80x9cbiochemical systemxe2x80x9d generally refers to a chemical interaction that involves molecules of the type generally found within living organisms. Such interactions include the full range of catabolic and anabolic reactions which occur in living systems including enzymatic, binding, signaling and other reactions. Biochemical systems of particular interest include, e.g., receptor-ligand interactions, enzyme-substrate interactions, cellular signaling pathways, transport reactions involving model barrier systems (e.g., cells or membrane fractions) for bioavailability screening, and a variety of other general systems.
Many methods have been described for the transport and direction of fluids, e.g., samples, analytes, buffers and reagents, within these microfluidic systems or devices. One method moves fluids within microfabricated devices by mechanical micropumps and valves within the device. See, Published U.K. Patent Application No. 2 248 891 (Oct. 18, 1990), Published European Patent Application No. 568 902 (May 2, 1992), U.S. Pat. No. 5,271,724 (Aug. 21, 1991) and U.S. Pat. No. 5,277,556 (Jul. 3, 1991). See also, U.S. Pat. No. 5,171,132 (Dec. 21, 1990) to Miyazaki et al. Another method uses acoustic energy to move fluid samples within devices by the effects of acoustic streaming. See, Published PCT Application No. 94/05414 to Northrup and White. A straightforward method applies external pressure to move fluids within the device. See, e.g., the discussion in U.S. Pat. No. 5,304,487 to Wilding et al.
Still another method uses electric fields, and the resulting electrokinetic forces, to move fluid materials through the channels of the microfluidic system. See, e.g., Published European Patent Application No. 376 611 (Dec. 30, 1988) to Kovacs, Harrison et al., Anal. Chem. (1992) 64:1926-1932 and Manz et al. J. Chromatog. (1992) 593:253-258, U.S. Pat. No. 5,126,022 to Soane. Electrokinetic forces have the advantages of direct control, fast response and simplicity. However, there are still some disadvantages with this method of operating a microfluidic system.
Present devices use a network of channels in a substrate of electrically insulating material. The channels connect a number of fluid reservoirs in contact with high voltage electrodes. To move fluid materials through the network of channels, specific voltages are simultaneously applied to the various electrodes. The determination of the voltage values for each electrode in a system becomes complex as one attempts to control the material flow in one channel without affecting the flow in another channel. For example, in a relatively simple arrangement of four channels intersecting in a cross with reservoirs and electrodes at the ends of the channels, an independent increase of fluid flow between two reservoirs is not merely a matter of increasing the voltage differences at the two reservoirs. The voltages at the other two reservoirs must also be adjusted if their original flow and direction are to be maintained. Furthermore, as the number of channels, intersections, and reservoirs are increased, the control of fluid through the channels become more and more complex.
Also, the voltages applied to the electrodes in the device can be high, i.e., up to a level supportive of thousands of volts/cm. Regulated high voltage supplies are expensive, bulky and are often imprecise and a high voltage supply is required for each electrode. Thus the cost of a microfluidic system of any complexity may become prohibitive.
The present invention solves or substantially mitigates these problems of electrokinetic transport in a microfluidic system which uses another electrical parameter, rather than voltage, to simplify the control of material flow through the channels of the system. A high throughput microfluidic system having direct, fast and straightforward control over the movement of materials through the channels of the microfluidic system with a wide range of applications, such as in the fields of chemistry, biochemistry, biotechnology and molecular biology and numerous other fields, is possible.
The present invention provides for a microfluidic system with a plurality of interconnected capillary channels and a plurality of electrodes at different nodes of the capillary channels which create electric fields in the capillary channels to electrokinetically move materials in a fluid through the capillary channels. In accordance with the present invention, the microfluidic system is operated by applying a voltage between a first electrode and a second electrode responsive to an electrical current between the first and second electrodes to move materials therebetween. Electrical current can give a direct measure of ionic flow through the channels of the microfluidic system. Besides current, other electrical parameters, such as power, may be also used.
Furthermore, the present invention provides for time-multiplexing the power supply voltages on the electrodes of the microfluidic system for more precise and efficient control. The voltage to an electrode can be controlled by varying the duty cycle of the connection of the electrode to the power supply, varying the voltage to the electrode during the duty cycle, or a combination of both. In this manner, one power supply can service more than one electrode.
The present invention also provides for the direct monitoring of the voltages within the channels in the microfluidic system. Conducting leads on the surface of the microfluidic system have widths sufficiently narrow in a channel to prevent electrolysis. The leads are connected to voltage divider circuits also on the surface of the substrate. The divider circuit lowers the read-out voltage of the channel node so that special high-voltage voltmeters are not required. The divider circuits are also designed to draw negligible currents from the channels thereby minimizing unwanted electrochemical effects, e.g., gas generation, reduction/oxidation reactions.
The invention as hereinbefore described may be put to a plurality of different uses, which are themselves inventive, for example, as follows:
The use of a substrate having at least one channel in which a subject material is transported electrokinetically, by applying a voltage between two electrodes associated with the channel in response to a current at the electrodes.
A use of the aforementioned invention, in which the substrate has a plurality of interconnected channels and associated electrodes, subject material being transported along predetermined paths incorporating one or more of the channels by the application of voltages to predetermined electrodes in response to a current at the electrodes.
The use of a substrate having at least one channel in which a subject material is transported electrokinetically by the controlled time dependent application of an electrical parameter between electrodes associated with the channel.
A use of the aforementioned invention, wherein the electrical parameter comprises voltage, current or power.
The use of an insulating substrate having a plurality of channels and a plurality of electrodes associated with the channels, the application of voltages to the electrodes causing electric fields in the channels, and at least one conductive lead on the substrate extending to a channel location so that an electric parameter at the channel location can be determined.
A use of the aforementioned invention, wherein the conductive lead has a sufficiently small width such that a voltage of less than 1 volt, and preferably less than 0.1 volt, is created across the conductive lead at the channel location.
A use of an insulating substrate having a plurality of interconnected capillary channels, a plurality of electrodes at different nodes of the capillary channels for creating electric fields in the capillary channels to move materials electrokinetically in a fluid through the capillary channels, a power supply connected to at least one of the electrodes, the power supply having a mixing block having a first input terminal for receiving a controllable reference voltage and a second input terminal, and an output terminal; a voltage amplifier connected to the mixing block output terminal, the voltage amplifier having first and second output terminals, the first output terminal connected to the at least one electrode; and a feedback block connected to the first output terminal of the voltage amplifier, the feedback block having an output terminal connected to the second input terminal of the mixing block so that negative feedback is provided to stabilize the power supply.
The use of the aforementioned invention, in which the feedback block is also connected to the second output terminal of the voltage amplifier, the feedback block generating a first feedback voltage responsive to a voltage at the first output terminal and a second feedback voltage responsive to an amount of current being delivered to the at least one electrode through the first output terminal, the feedback block having a switch for passing the first or second feedback voltage to the mixing block responsive to a control signal so that the power supply is selectably stabilized by voltage or current feedback.
The use of a power supply for connection to at least one electrode of a microfluidic system in which the power supply has a mixing block having a first input terminal for receiving a controllable reference voltage and a second input terminal, and an output terminal; a voltage amplifier connected to the mixing block output terminal, the voltage amplifier having first and second output terminals, the first output terminal connected to the at least one electrode; and a feedback block connected to the first and second output terminals of the voltage amplifier and to the second input terminal of the mixing block, the feedback block generating a first feedback voltage responsive to a voltage at the first output terminal and a second feedback voltage responsive to an amount of current being delivered to the at least one electrode through the first output terminal, the feedback block having a switch for passing the first or second feedback voltage to the mixing block responsive to a control signal so that the power supply is selectably stabilized in voltage or current by negative feedback.
The use of a microfluidic system in which a substrate has a plurality of interconnected capillary channels, a plurality of electrodes at different nodes of the capillary channels for creating electric fields in the capillary channels to move materials electrokinetically in a fluid through the capillary channels, and a plurality of power supplies connected to each one of the electrodes, each of the power supplies capable of selectively supplying a selected voltage and a selected amount of current as a source or sink to the connected electrodes.